In an active matrix type LCD device, all pixels are driven by a plurality of active elements having non-liner characteristics, each of which is disposed in each of pixels arranged in matrix shape. As the active elements, TFT elements are generally used.
In the LCD device, optical display characteristics are dependent on TFT elements, liquid crystal material, cell gaps, color filters and the like. Accordingly, as the TFT LCD device is used for a long time, flickers or image stickings deteriorating display characteristic may occur.
To reduce flickers, various methods have been proposed. Examples of the conventional methods of reducing flickers are disclosed in U.S. Pat. No. 5,253,091 issued to Kimura et al., on October, 1993 and entitled Liquid Crystal Display Having Reduced Flickers,” and U.S. Pat. No. 5,436,747 issued to Suzuki et al., on July, 1995 and entitled “Reduced Flicker Liquid Crystal Display.”
A general LCD device includes a TFT substrate having a plurality of pixel electrodes and TFT elements, an opposite color filter substrate having common electrodes and color filters, and a liquid crystal material therebetween.
In the LCD device, a plurality of TFT elements, each of which is disposed in a pixel, supply voltage to the common electrodes formed on the color filter substrate and the pixel electrodes formed on the TFT substrate to control electric fields which are to be applied to the liquid crystal. When the pixel electrodes and the common electrodes are applied with voltages by the operation of the corresponding TFT elements, the molecules of the liquid crystal material change their orientations in response to the electric fields due to the potential difference between the pixel electrodes and the common electrodes. At this time, the electric field between two electrodes is generally controlled to periodically change its direction.
For example, signal voltage supplied to the pixel electrodes through the TFT elements is periodically inverted with respect to common electrode voltage supplied to the common electrodes. At this time, if actual values of inverted signal voltages of positive and negative against the common electrode voltage are same, flickers and afterimages or image stickings do not occur. However, if the virtual values of the positive and negative voltages are different from each other, electric fields having elements of direct current may be applied between two electrodes to generate image stickings. Also, if positive and negative voltages to same gray scale are not symmetrical to each other with respect to the common electrode voltage, brightness of each pixel may come to be different according to each of the positive and negative voltages and thereby flickers occur. Even though at first the common electrode voltage has been correctly modulated not to impose the elements of direct current and the like, components of the LCD device such as the TFT elements, color filters, and a protecting sheet physically change as it is used. Accordingly, the common electrode voltage may be deviated from optimum condition and thereby increase flickers. Particularly, a large size LCD of a high definition may exponentially increase an amount of visually recognized flickers.